Why do I see edges partially see-through on back face (in viewport)?
I don't really know the correct answer to this one, I think this is a limitation of the current opengl viewport rendering engine. Would like to hear from one of the devs about it, if it is actually fixable or a bug, or if anyone else knows any other way to fix it.

Only way to remedy the situation I found is to adjust viewport clipping. It's a workaround with its own side effects but can minimize the issue.

In the 3d view properties shelf N key, under the View panel there is a setting for viewpor clipping, star and end. Try bringing the two values closer together like increasing clip start and/or increasing clip end but keeping them in a usable state for your scene.

Edit: Worked well on my end


Ok, missed one detail, if you are in camera view mode, then you need to adjust the clipping settings in the camera's Object Data Settings in the Properties window, not viewport clipping as it has no effect while in camera view mode.